Below, conventional techniques and problems thereof will be explained.
Generally, coil springs repeatedly undergo compressive stress. With regard to this, the surface of the coil springs are typically treated by shot peening to increase the internal stress thereof.
In conventional shot peening apparatuses which treat the surfaces of coil springs through shot peening processes to increase the internal stress thereof, coil springs are placed on a pair of rollers which are rotated. The coil springs are rotated by the rotation of the rollers. Simultaneously, a roller moving device, which is coupled to the rollers, is operated to pass the rollers and the coil springs through a shot room. Thereby, the surfaces of the coil springs are treated by shot peening.
Here, when treating coil springs through the shot peening process, if the coil springs are in the compressed state, the internal stress of the coil springs can be further increased. However, in the case of the conventional shot peening apparatuses, coil springs are only rotated in the original states thereof without being compressed, when they are treated by shot peening. Therefore, there is a problem in that high quality products, that is, coil springs having increased internal stress, cannot be produced.
Furthermore, the conventional shot peening apparatuses allow coil springs to be freely rotated on the rollers without holding the opposite ends of the coil springs. Thus, a typical coil spring having a basic cylindrical shape is evenly treated by shot peening throughout the entire surface thereof. However, in the case of a side load coil spring, which is increased in diameter from the opposite ends thereof to the medial portion thereof, when the side load coil spring is rotated, the longitudinal axis thereof cannot maintain a horizontal state, in other words, the coil spring suffers irregular seesaw motion, in which the opposite ends thereof are moved upwards and downwards. Therefore, the coil spring cannot be evenly treated by shot peening through the entire surface thereof.